1. Field of the Invention
The present invention relates to a workpiece dividing method for dividing a workpiece such as a semiconductor wafer.
2. Description of the Related Art
In a semiconductor device fabrication process, a plurality of crossing division lines called streets are formed on the front side of a workpiece such as a semiconductor wafer and an optical device wafer to thereby partition a plurality of regions where a plurality of circuits such as semiconductor devices and optical devices are respectively formed. The workpiece is cut along the division lines to divide the regions where the circuits are formed from each other, thus obtaining individual semiconductor chips. As a method of dividing the workpiece along the division lines, a laser processing method using a pulsed laser beam having a transmission wavelength to the workpiece is known (see Japanese Patent No. 3408805, for example).
In this laser processing method (dividing method), the pulsed laser beam having a transmission wavelength is applied to the workpiece from one side thereof along the division lines in the condition where the focal point of the pulsed laser beam is set inside the workpiece, thereby continuously forming a modified layer inside the workpiece along each division line. Thereafter, an external force is applied to the workpiece along the division lines where the modified layers are respectively formed to reduce the strength, thereby dividing the workpiece into semiconductor chips.
In the case of forming the modified layers in the method of dividing the workpiece into the semiconductor chips, there are the following cases. That is, (1) there is a case that the laser beam is required to be applied from the back side of the workpiece where the circuits are not formed, so as to form the modified layers in the workpiece along the streets having a small pitch. (2) It is difficult to handle the workpiece having the modified layers in its easily breakable condition and it is therefore desirable to avoid the step of replacing a tape attached to the workpiece after forming the modified layers. (3) In the step of picking up the semiconductor chips after dividing the workpiece, the front side of each semiconductor chip where the circuit is formed must be exposed. In consideration of the above cases, it is sometimes desired to apply the laser beam to the workpiece through a dicing tape attached to the back side of the workpiece, thus forming the modified layers in the workpiece.